The invention relates to magnetic resonance (MR), and more particularly relates to MR angiography (MRA). In its most immediate sense, the invention relates to three-dimensional MRA in which contrast agent is used.
In MRA, and particularly in three-dimensional MRA, it is advantageous for a clinician (usually a radiologist) to be able to distinguish arteries from veins. Existing methodology for doing this has proven unsatisfactory.
In conventional MRA (which does not use contrast agents), venous or arterial signals are selectively eliminated during image acquisition by presaturating the venous or arterial blood flow and thereby preventing it from producing an MR signal. The typical result is an MR angiogram depicting only arteries, or only veins. The effective shortening of the T1 relaxation time in blood which produces a high signal-to-noise ratio in contrast-enhanced MRA also causes such conventional presaturation techniques to fail. Because of this, to distinguish between arteries and veins, contrast-enhanced MRA data is typically acquired in two steps, the first being carried out to acquire an enhanced arterial image and the second being carried out when both arteries and veins are enhanced. The first image displays the arteries, and a subtraction image formed between the first image and the second image displays the veins. Each of these images must be acquired rapidly; the first image must capture the peak of the arterial bolus, and the second must be carried out before the venous enhancement diminishes, and in the case of conventional extravascular agents, before the surrounding tissue is significantly enhanced. Because each of the images must be acquired quickly, the images necessarily have low spatial resolution.
Because the subtraction image only relates to the state of the patient's circulatory system at two particular times, such a methodology provides only a limited capability of distinguishing between arteries and veins and is highly sensitive to the timing and speed of data acquisition relative to the injection of the contrast agent. If this timing is miscalculated, or if the acquisition lasts too long, the acquisition does not acquire a purely arterial phase, but rather a combination of arterial and venous phases together. Furthermore, if some veins have not yet enhanced by the time of the second acquisition, they will not appear in the subtracted image.
It would be advantageous to provide improved methodology for acquiring three-dimensional MRA data from a patient's arteries and veins within a volume of interest (VOI) and for displaying an image reconstructed from such data in which it would be possible to visually distinguish the arteries from the veins.
It is therefore an object of the invention to provide a method that would make it possible to distinguish arteries from veins in a three-dimensional MRA image.
The invention proceeds from the realization that by administering a bolus of an MR contrast agent into the patient's circulatory system and by tracking the position of the bolus as a function of time, it is possible to distinguish arteries from veins. This is because the bolus normally passes through the patient's circulatory system along a route that is known in advance, and the position of the bolus within the circulatory system can therefore in the normal case be correlated with time post administration. For this reason, the progress of the contrast agent through the patient's circulatory system as a function of time can therefore be used to distinguish arteries from veins. Indeed, even if the blood does not flow along the expected path (as can be the case when e.g. the patient's heart is malformed) the actual flow path can be mapped out as a function of time, thereby providing the physician with blood flow images that are outside conventional "venous" or "arterial" categories.
In accordance with the invention, this time-based information is used to scale the intensity of voxels in a three-dimensional MR image. Put another way, in accordance with the invention, for each voxel within the VOI, the enhancement of that voxel as a function of time is determined, and individual voxels in a three-dimensional MR image of the VOI are visually emphasized and de-emphasized in accordance with this determination and the requirements of the physician or technician.
The preferred embodiment of the invention further proceeds from the realization that if an intravascular contrast agent is used, a high resolution acquisition of longer duration can be run after the contrast agent has reached an equilibrium state in the bloodstream. The time-based information is used to scale the displayed intensities of voxels in a high-resolution MR angiogram, and the image intensities of individual voxels in the high-resolution MR angiogram can be manipulated so as to visually distinguish arteries from veins. While this scaling step is particularly advantageous because of the higher resolution of the diagnostic image, it is not necessary. Even without an intravascular contrast agent and a high-resolution MR angiogram, the time-based information is sufficient to characterize the path of blood flow and to thereby distinguish and depict arteries and veins.
In accordance with another of its aspects, the invention resides in a) administering a bolus of contrast agent to the patient's circulatory system in such a manner as to enhance the vascular structure, b) acquiring three-dimensional MR image data from the VOI in such a manner as to register, as a function of time, movement of the contrast agent through the vascular structure, c) scaling voxels in a three-dimensional MR image of the VOI in accordance with the individual sections to be selectively emphasized, and d) displaying the three-dimensional MR image using image values taken from the scaled acquired MR image data. Advantageously, a high-resolution three-dimensional MR image of the VOI is acquired after the contrast agent has reached equilibrium in the VOI, the scaling step is applied to the image data in the high-resolution three-dimensional MR image, and the displaying step includes the step of displaying a Maximum Intensity Projection ("MIP") of the three-dimensional MR image in which the image values have been scaled.
In accordance with another aspect of the invention, a bolus of contrast agent is administered to the patient's circulatory system in such a manner as to enhance the patient's blood vessels within the VOI. Then, in accordance with this aspect of the invention, a plurality of three-dimensional MR datasets (these will be referred to herein as "dynamic MR datasets") are acquired from the VOI, beginning with the administration of the contrast medium and continuing for a sufficiently long time to reflect contrast agent enhancement of all arterial and venous blood vessels within the VOI. These dynamic MR datasets, taken as a whole, in effect provide time-based information which, as explained above, can be used to distinguish arteries from veins (and indeed can be used to distinguish between different parts of a single artery or vein).
Advantageously, and if an intravascular contrast agent is available, a three-dimensional MR angiogram of the patient's VOI is acquired after the contrast agent has reached equilibrium. This in effect provides a high-quality three-dimensional image of the patient's vasculature. Then, for each voxel within the VOI, enhancement of that voxel as a function of time post administration of the contrast agent is computed based on the information derived from the dynamic 3D datasets. After such computation has taken place, parameters (advantageously, time-to-peak-enhancement, magnitude of peak enhancement, slope of signal enhancement as a function of time) that distinguish enhancement of voxels relating to the patient's arteries from enhancement of voxels relating to the patient's veins are selected and the intensity of each voxel in the three-dimensional MR angiogram is scaled in accordance with the selected parameters. Then, advantageously in accordance with the preferred embodiment of the invention, a maximum intensity projection ("MIP") reconstruction of the VOI is generated. Although an MIP reconstruction is presently preferred, it is not necessary; it may alternatively be advantageous to use some form of 3D surface rendering instead.
Advantageously, and in accordance with the preferred embodiment of the invention, the physician or technologist carries out the scaling step interactively while viewing the MIP reconstruction. This permits the physician or technologist to so adjust the display as to emphasize only the particular structure (vein, artery, or part(s) thereof) of interest. Further advantageously, the dynamic MR datasets are acquired rapidly to provide high temporal resolution. When so acquired, the dynamic MR datasets are of relatively low spatial resolution. If the contrast agent is of the intravascular type, a high-resolution three-dimensional MR angiogram can be acquired over a longer time. The voxel sizes of the dynamic MR datasets and the high-resolution MR angiogram are effectively normalized by spatially interpolating the dynamic MR datasets to correspond to the spatial resolution of the high resolution angiogram. In this manner, a signal enhancement curve is generated for every voxel in the high resolution angiogram.